"Cricket was by far the biggest sport [in the USA] in this period," Tim Lockley, an expert in American history at Warwick University, told the Guardian in 1999. "Then the civil war started in 1861, just when it was reaching its peak of popularity. The sport became a victim of that war." (source: History.SE answer)

Why exactly did Civil War have this negative effect on cricket's popularity in USA?

3 Answers
3

In spite of all this American growth in the game, it was slowly losing ground to a newcomer. In many cities, local cricket clubs were contributing to their own demise by encouraging crossover to the developing game of baseball. After the United States Civil War the Cincinnati Red Stockings brought a talented young bowler from the St. George's Cricket Club in New York to serve as a player and manager of the team. Harry Wright applied the "scientific" batting and specialized placement of fielders that he had learned in cricket to his new sport. This development was instrumental in creating the Cincinnati team's undefeated 1869 season. It also helped to secure the place of baseball as one of the most popular sports in the country.

It may have been during the Civil War that baseball secured its place as America's game. An army making a brief stop at a location could easily organise a game of baseball on almost any clear patch of ground, whilst cricket required a carefully prepared pitch. Baseball began to poach players and administrators from the world of cricket. Nick Young, who served for 25 years as the president of the National League, was originally a successful cricketer. It was not until the Civil War that he took up baseball because "it looked like cricket for which his soul thirsted." It has been suggested that the fast-paced quick play of baseball was more appealing to Americans than the technical slower game of cricket. This natural tendency toward baseball was compounded by terrible American defeats at the hands of a traveling English side in 1859, which may have caused Americans to think that they would never be successful at this English game. By the end of the Civil War, most cricket fans had given up their hopes of broad-based support for the game. Baseball filled the role of the "people's game" and cricket became an amateur game for gentlemen.

So, here we have a few major reasons for the crossover:

Many famous cricket players were switching over to baseball. This could be attributed to the next point.

Baseball was quicker paced than cricket. Because, there was no clock, there was no defensive batting, making it more exciting.

Baseball didn't need a special field. This made it much easier for soldiers to play on the move. Obviously, afterwards, they would bring it home and popularize it.

Baseball suited war-time needs. It was quick, easy to learn, and required little in the way of equipment or facilities. No pitch was needed - just four sacks thrown on the ground, a simple bat and ball.

My opinion is that the third reason had the most effect. With hundreds of thousands being readily able to play baseball instead of cricket, they exerted a large influence of the sports played at home when they returned.

Yeah, the third reason is also the prominent reason why soccer is so popular over, say, rugby or basketball: you can play it easily pretty much anywhere, even in incredibly irregular oddly-shaped fields, regardless of their material or condition…
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LohorisFeb 18 at 8:52

It'a a (partially) false premise. Cricket was popular with Americans (at least those with high social status) long after the Civil War.

While the increasing popularity of baseball did present a formidable
challenge to American cricket, the two games existed comfortably
side-by-side throughout the 1850s and 60s. It was not uncommon, in
fact, for cricket and baseball teams to challenge one another to
matches in their rival’s sport. (source)

However much the American elite enjoyed playing cricket, the sport never developed the kind of infrastructure that leads to a mass fan base: frequent matches with large crowds and intense rivalries. Baseball, however

. . . was later blessed by a cadre of brilliant entrepreneurs
determined to make it the “nation’s pastime.” One such person was A.
G. Spalding, star player, manager, league organizer, and sports
manufacturer. To call Spalding an impresario or a marketing genius
would be a bit of an understatement. He engaged in every part of the
game, from promoting star players and intercity rivalries to
squelching nascent efforts at labor organization among players. (source)

I have declared that Cricket is a genteel game. It is. Our British
Cricketer, having finished his day’s labor at noon, may don his
negligee shirt, his white trousers, his gorgeous hosiery and his
canvas shoes, and sally forth to the field of sport, with his
sweetheart on one arm and his Cricket bat under the other, knowing
that he may engage in his national pastime without soiling his linen
or neglecting his lady . . .

Cricket is a gentle pastime. Base Ball is War! Cricket is an Athletic
Sociable, played and applauded in a conventional, decorous and English
manner. Base Ball is an Athletic Turmoil, played and applauded in an
unconventional, enthusiastic and American manner.

I think demographics played some part, too. Baseball was very popular in New York, even called at one time "The New York Game." With the large number of soldiers from New York serving in the Federal army, it was widely spread.